1. Field of the Invention
This invention relates to pollution control, namely filtering of particulate matter, and more specifically, to filtering of flyash and other particulates from flue gas.
2. Description of the Prior Art
Currently, there are approximately 1200 coal-fired utility power plants representing 330,000 MWe of generating capacity that are equipped with electrostatic precipitators. Present precipitators typically remove 90-99.9% of the flyash in the flue gas. However, existing and pending regulations to control sulfur dioxide emissions from the flue gas require utilities to switch fuel types (such as from high to low sulfur coal), or add sulfur dioxide control upstream of the precipitators. Fuel switching and sulfur control upstream of the precipitators generally modify flyash properties, reduce precipitator collection efficiency, and increase stack particulate emissions. In addition, particulate emissions standards are getting increasingly stringent. Faced with these increasingly strict environmental requirements, utilities are looking for low cost retrofits to upgrade the performance of their precipitators.
It is well known in the art how to build and use electrostatic precipitators. It is also known in the art how to build and use a barrier filter such as a baghouse. Further, it is known in the art how to charge particles and that charged particles may be collected in a barrier filter with lower pressure drop and emissions than uncharged particles collected for the same filtration velocity.
Electric power utility companies are looking for ways to upgrade their precipitators. One approach would be to replace the existing under-performing precipitator with a baghouse or barrier filter or conventional design which are generally accepted as an alternative to precipitators for collecting flyash from flue gas. Conventional designs can be categorized as low-ratio baghouses (reverse-gas, sonic-assisted reverse-gas, and shake-deflate) which generally operate at filtration velocities of 0.76 to 1.27 centimeters per second (1.5 to 2.5 ft/min), also defined as air-to-cloth ratio, volumetric flow rate of flue gas per unit of effective filter area, or (cubic feet of flue gas flow/min/square foot of filtering area), and high-ratio pulse-jet baghouses which generally operate at 1.52 to 2.54 centimeters per second (3 to 5 ft/min). Baghouses generally have very high collection efficiencies (greater than 99.9%) independent of flyash properties. However, because of their low filtration velocities, they are large, require significant space, are costly to build, and are unattractive as replacements for existing precipitators. Reducing their size by increasing the filtration velocity across the filter bags will result in unacceptably high pressure drops and outlet particulate emissions. There is also potential for "blinding" the filter bags--a condition where particles are embedded deep within the filter and reduce flow drastically.
In U.S. Pat. No. 3,915,676 which issued on Oct. 28, 1975 to Reed et al., an electrostatic dust collector is disclosed where the dirty gas is moved through an electrostatic precipitator to remove most of the particulate matter. The gas stream then passes through a filter having a metal screen and dielectric material wherein an electric field is applied to the filter which permits a more porous material to be used in the filter. The filter is of formacious and dielectric material to collect the charged fine particles. The filter and precipitator are designed in a concentric tubular arrangement with the dirty gas passing from the center of the tubes outward.
In U.S. Pat. No. 4,147,522 which issued on Apr. 3, 1979 to Gonas et al., the dirty gas stream passes through a tubular precipitator and then directly into a filter tube in series with the precipitator tube. The particles are electrically charged and are deposited on the fabric filter which is of neutral potential with regard to the precipitator. The major portion of the particles are however deposited in the electrostatic precipitator. No electric field is applied to the fabric filter. Precipitator and filter tube are cleaned simultaneously by a short burst of air.
In U.S. Pat. No. 4,354,858 which issued on Oct. 19, 1982 to Kumar et al., electrically charged particles in a gas stream are filtered from the stream by a filter medium which includes a porous cake composed of electrically charged particulates previously drawn from the gas stream and collected on a foraminous support structure.
In U.S. Pat. No. 4,357,151 which issued on Nov. 2, 1982 to Helfritch et al., an apparatus is disclosed which first moves dirty gas through a corona discharge electrodes located in the spaced between mechanical filters of the cartridge type having a filter medium of foraminous dielectric material such as pleated paper. The zone of corona discharge in the dirty gas upstream of the filter results in greater particle collection efficiency and lower pressure drop in the mechanical filters.
In U.S. Pat. No. 4,411,674 which issued on Oct. 25, 1983 to Forgac, a cyclone separator is disclosed wherein a majority of the dust is removed from dirty air in a conventional fashion followed by a bag filter. The bottoms of the filter bags have open outlets for delivering dust into a bottom chamber. The particulates are continuously conducted out of the bag filter apparatus for recirculation back to the cyclone separator.
In all the above patents, the inventors are looking for ways to reduce pressure drop and emissions across a barrier filter by precharging or mechanical precollection of the particles in the gas stream.